What is characteristic of an Imperfect Authentic Cadence (IAC)?

An Imperfect Authentic Cadence (IAC) is characterized by a V-I progression in which the soprano voice does not resolve on the tonic. This can often result in the leading tone moving to another note rather than the tonic, giving a less conclusive sound compared to a Perfect Authentic Cadence (PAC), where both chords are in root position and the highest voice lands on the tonic note. This unresolved nature of the soprano voice in an IAC creates a sense of tension or continuation, differentiating it from a PAC.

In the context of the other provided choices, a V-I progression with both chords in root position describes a Perfect Authentic Cadence, not an IAC; the IV-I progression is more typical of plagal cadences rather than an IAC; and the description of a V chord approached by other chords doesn’t capture the essential harmonic resolution that defines an IAC. Thus, the distinguishing feature of the soprano melody not resolving to the tonic highlights why the answer is correct.